Zak, welcome. Yes, in a 5.1 setup the QS8s are sides, not "rears". If they have to be mounted on the back wall because of the wiring, at least they should be separated as much as possible, to within a few inches of the side walls.
I only have 6" of wire protruding from the rear wall plates so I'm really limited for placement options.
I just read the QS8 manual and although it's not ideal it should be ok on the rear wall:
"Quadpolar surround speakers in a 5.1 system will sound their best and
produce the greatest sense of envelopment in the surround soundfield if
they are to each side of the listening area (see diagram) rather than on
the rear walls, and from 1 to 5 feet above the level of seated listener’s
ears.
However if your room doesn’t permit side-wall placement of the
surrounds, the QS models will work very effectively on the rear wall
behind the listening area."
Zak
